Integral 3D graphene-carbon hybrid foam and devices containing same

ABSTRACT

Provided is an integral 3D graphene-carbon hybrid foam composed of multiple pores and pore walls, wherein the pore walls contain single-layer or few-layer graphene sheets chemically bonded by a carbon material having a carbon material-to-graphene weight ratio from 1/100 to 1/2, wherein the few-layer graphene sheets have 2-10 layers of stacked graphene planes having an inter-plane spacing d 002  from 0.3354 nm to 0.40 nm and the graphene sheets contain a pristine graphene material having essentially zero % of non-carbon elements, or a non-pristine graphene material having 0.01% to 25% by weight of non-carbon elements wherein said non-pristine graphene is selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, doped graphene, chemically functionalized graphene, or a combination thereof. Also provided are a process for producing the hybrid form, products containing the hybrid foam, and its applications.

FIELD OF THE INVENTION

The present invention relates generally to the field of carbon/graphitefoams and, more particularly, to a new form of porous graphitic materialherein referred to as an integral 3D graphene-carbon hybrid foam, aprocess for producing same, products containing same, and a method ofoperating the product.

BACKGROUND OF THE INVENTION

Carbon is known to have five unique crystalline structures, includingdiamond, fullerene (0-D nano graphitic material), carbon nano-tube orcarbon nano-fiber (1-D nano graphitic material), graphene (2-D nanographitic material), and graphite (3-D graphitic material). The carbonnano-tube (CNT) refers to a tubular structure grown with a single wallor multi-wall. Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs)have a diameter on the order of a few nanometers to a few hundrednanometers. Their longitudinal, hollow structures impart uniquemechanical, electrical and chemical properties to the material. The CNTor CNF is a one-dimensional nano carbon or 1-D nano graphite material.

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A.Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

A single-layer graphene sheet is composed of carbon atoms occupying atwo-dimensional hexagonal lattice. Multi-layer graphene is a plateletcomposed of more than one graphene plane. Individual single-layergraphene sheets and multi-layer graphene platelets are hereincollectively called nano graphene platelets (NGPs) or graphenematerials. NGPs include pristine graphene (essentially 99% of carbonatoms), slightly oxidized graphene (<5% by weight of oxygen), grapheneoxide (≧5% by weight of oxygen), slightly fluorinated graphene (<5% byweight of fluorine), graphene fluoride ((≧5% by weight of fluorine),other halogenated graphene, and chemically functionalized graphene.

NGPs have been found to have a range of unusual physical, chemical, andmechanical properties. For instance, graphene was found to exhibit thehighest intrinsic strength and highest thermal conductivity of allexisting materials. Although practical electronic device applicationsfor graphene (e.g., replacing Si as a backbone in a transistor) are notenvisioned to occur within the next 5-10 years, its application as anano filler in a composite material and an electrode material in energystorage devices is imminent. The availability of processable graphenesheets in large quantities is essential to the success in exploitingcomposite, energy, and other applications for graphene.

Our research group was among the first to discover graphene [B. Z. Jangand W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patent applicationSer. No. 10/274,473, submitted on Oct. 21, 2002; now U.S. Pat. No.7,071,258 (Jul. 4, 2006)]. The processes for producing NGPs and NGPnanocomposites were recently reviewed by us [Bor Z. Jang and A Zhamu,“Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: AReview,” J. Materials Sci. 43 (2008) 5092-5101]. Four main prior-artapproaches have been followed to produce NGPs. Their advantages andshortcomings are briefly summarized as follows:

A Review on Production of Isolated Nano Graphene Plates or Sheets (NGPs)Approach 1: Chemical Formation and Reduction of Graphene Oxide (GO)

The first approach (FIG. 1) entails treating natural graphite powderwith an intercalant and an oxidant (e.g., concentrated sulfuric acid andnitric acid, respectively) to obtain a graphite intercalation compound(GIC) or, actually, graphite oxide (GO). [William S. Hummers, Jr., etal., Preparation of Graphitic Oxide, Journal of the American ChemicalSociety, 1958, p. 1339.] Prior to intercalation or oxidation, graphitehas an inter-graphene plane spacing of approximately 0.335 nm(L_(d)=½d₀₀₂=0.335 nm). With an intercalation and oxidation treatment,the inter-graphene spacing is increased to a value typically greaterthan 0.6 nm. This is the first expansion stage experienced by thegraphite material during this chemical route. The obtained GIC or GO isthen subjected to further expansion (often referred to as exfoliation)using either a thermal shock exposure or a solution-based,ultrasonication-assisted graphene layer exfoliation approach.

In the thermal shock exposure approach, the GIC or GO is exposed to ahigh temperature (typically 800-1,050° C.) for a short period of time(typically 15 to 60 seconds) to exfoliate or expand the GIC or GO forthe formation of exfoliated or further expanded graphite, which istypically in the form of a “graphite worm” composed of graphite flakesthat are still interconnected with one another. This thermal shockprocedure can produce some separated graphite flakes or graphene sheets,but normally the majority of graphite flakes remain interconnected.Typically, the exfoliated graphite or graphite worm is then subjected toa flake separation treatment using air milling, mechanical shearing, orultrasonication in water. Hence, approach 1 basically entails threedistinct procedures: first expansion (oxidation or intercalation),further expansion (or “exfoliation”), and separation.

In the solution-based separation approach, the expanded or exfoliated GOpowder is dispersed in water or aqueous alcohol solution, which issubjected to ultrasonication. It is important to note that in theseprocesses, ultrasonification is used after intercalation and oxidationof graphite (i.e., after first expansion) and typically after thermalshock exposure of the resulting GIC or GO (after second expansion).Alternatively, the GO powder dispersed in water is subjected to an ionexchange or lengthy purification procedure in such a manner that therepulsive forces between ions residing in the inter-planar spacesovercome the inter-graphene van der Waals forces, resulting in graphenelayer separations.

There are several major problems associated with this conventionalchemical production process:

-   -   (1) The process requires the use of large quantities of several        undesirable chemicals, such as sulfuric acid, nitric acid, and        potassium permanganate or sodium chlorate.    -   (2) The chemical treatment process requires a long intercalation        and oxidation time, typically 5 hours to five days.    -   (3) Strong acids consume a significant amount of graphite during        this long intercalation or oxidation process by “eating their        way into the graphite” (converting graphite into carbon dioxide,        which is lost in the process). It is not unusual to lose 20-50%        by weight of the graphite material immersed in strong acids and        oxidizers.    -   (4) The thermal exfoliation requires a high temperature        (typically 800-1,200° C.) and, hence, is a highly        energy-intensive process.    -   (5) Both heat- and solution-induced exfoliation approaches        require a very tedious washing and purification step. For        instance, typically 2.5 kg of water is used to wash and recover        1 gram of GIC, producing huge quantities of waste water that        need to be properly treated.    -   (6) In both the heat- and solution-induced exfoliation        approaches, the resulting products are GO platelets that must        undergo a further chemical reduction treatment to reduce the        oxygen content. Typically even after reduction, the electrical        conductivity of GO platelets remains much lower than that of        pristine graphene. Furthermore, the reduction procedure often        involves the utilization of toxic chemicals, such as hydrazine.    -   (7) Furthermore, the quantity of intercalation solution retained        on the flakes after draining may range from 20 to 150 parts of        solution by weight per 100 parts by weight of graphite flakes        (pph) and more typically about 50 to 120 pph. During the        high-temperature exfoliation, the residual intercalate species        retained by the flakes decompose to produce various species of        sulfuric and nitrous compounds (e.g., NO_(x) and SO_(x)), which        are undesirable. The effluents require expensive remediation        procedures in order not to have an adverse environmental impact.        The present invention was made to overcome the limitations or        problems outlined above.

Approach 2: Direct Formation of Pristine Nano Graphene Platelets

In 2002, our research team succeeded in isolating single-layer andmulti-layer graphene sheets from partially carbonized or graphitizedpolymeric carbons, which were obtained from a polymer or pitch precursor[B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/274,473, submitted on Oct. 21, 2002; now U.S.Pat. No. 7,071,258 (Jul. 4, 2006)]. Mack, et al [“Chemical manufactureof nanostructured materials” U.S. Pat. No. 6,872,330 (Mar. 29, 2005)]developed a process that involved intercalating graphite with potassiummelt and contacting the resulting K-intercalated graphite with alcohol,producing violently exfoliated graphite containing NGPs. The processmust be carefully conducted in a vacuum or an extremely dry glove boxenvironment since pure alkali metals, such as potassium and sodium, areextremely sensitive to moisture and pose an explosion danger. Thisprocess is not amenable to the mass production of NGPs. The presentinvention was made to overcome the limitations outlined above.

Approach 3: Epitaxial Growth and Chemical Vapor Deposition of NanoGraphene Sheets on Inorganic Crystal Surfaces

Small-scale production of ultra-thin graphene sheets on a substrate canbe obtained by thermal decomposition-based epitaxial growth and a laserdesorption-ionization technique. [Walt A. DeHeer, Claire Berger, PhillipN. First, “Patterned thin film graphite devices and method for makingsame” U.S. Pat. No. 7,327,000 B2 (Jun. 12, 2003)] Epitaxial films ofgraphite with only one or a few atomic layers are of technological andscientific significance due to their peculiar characteristics and greatpotential as a device substrate. However, these processes are notsuitable for mass production of isolated graphene sheets for compositematerials and energy storage applications. The present invention wasmade to overcome the limitations outlined above.

Another process for producing graphene, in a thin film form (typically<2 nm in thickness), is the catalytic chemical vapor deposition process.This catalytic CVD involves catalytic decomposition of hydrocarbon gas(e.g. C₂H₄) on Ni or Cu surface to form single-layer or few-layergraphene. With Ni or Cu being the catalyst, carbon atoms obtained viadecomposition of hydrocarbon gas molecules at a temperature of800-1,000° C. are directly deposited onto Cu foil surface orprecipitated out to the surface of a Ni foil from a Ni—C solid solutionstate to form a sheet of single-layer or few-layer graphene (less than 5layers). The Ni- or Cu-catalyzed CVD process does not lend itself to thedeposition of more than 5 graphene planes (typically <2 nm) beyond whichthe underlying Ni or Cu layer can no longer provide any catalyticeffect. The CVD graphene films are extremely expensive.

Approach 4: The Bottom-Up Approach (Synthesis of Graphene from SmallMolecules)

Yang, et al. [“Two-dimensional Graphene Nano-ribbons,” J. Am. Chem. Soc.130 (2008) 4216-17] synthesized nano graphene sheets with lengths of upto 12 nm using a method that began with Suzuki-Miyaura coupling of1,4-diiodo-2,3,5,6-tetraphenyl-benzene with 4-bromophenylboronic acid.The resulting hexaphenylbenzene derivative was further derivatized andring-fused into small graphene sheets. This is a slow process that thusfar has produced very small graphene sheets. The present invention wasmade to overcome the limitations outlined above.

Hence, an urgent need exists to have a graphene production process thatrequires a reduced amount of undesirable chemical (or elimination ofthese chemicals all together), shortened process time, less energyconsumption, lower degree of graphene oxidation, reduced or eliminatedeffluents of undesirable chemical species into the drainage (e.g.,sulfuric acid) or into the air (e.g., SO₂ and NO₂). The process shouldbe able to produce more pristine (less oxidized and damaged), moreelectrically conductive, and larger/wider graphene sheets. Furthermore,one should be able to readily make these graphene sheets into a foamstructure.

Our recent research has yielded a process for chemical-free productionof isolated nano graphene platelets that is novel in that is does notfollow the established methods for production of nano graphene plateletsoutlined above. In addition, the process is of enhanced utility in thatit is cost effective, and provided novel graphene materials withsignificantly reduced environmental impact. Furthermore, as hereindisclosed, we have combined the chemical-free production of graphene andthe formation of a graphene-carbon hybrid form into one singleoperation.

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers) pristine graphene,graphene oxide, reduced graphene oxide (RGO), graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene,doped graphene (e.g. doped by B or N). Pristine graphene has essentially0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight.Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.Other than pristine graphene, all the graphene materials have 0.001%-50%by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.).These materials are herein referred to as non-pristine graphenematerials. The presently invented graphene-carbon foam can containpristine or non-pristine graphene and the invented method allows forthis flexibility.

A Review on Production of Graphene Foams

Generally speaking, a foam or foamed material is composed of pores (orcells) and pore walls (a solid material). The pores can beinterconnected to form an open-cell foam. A graphene foam is composed ofpores and pore walls that contain a graphene material. There are threemajor methods of producing graphene foams:

The first method is the hydrothermal reduction of graphene oxidehydrogel that typically involves sealing graphene oxide (GO) aqueoussuspension in a high-pressure autoclave and heating the GO suspensionunder a high pressure (tens or hundreds of atm) at a temperaturetypically in the range of 180-300° C. for an extended period of time(typically 12-36 hours). A useful reference for this method is givenhere: Y. Xu, et al. “Self-Assembled Graphene Hydrogel via a One-StepHydrothermal Process,” ACS Nano 2010, 4, 4324-4330. There are severalmajor issues associated with this method: (a) The high pressurerequirement makes it an impractical method for industrial-scaleproduction. For one thing, this process cannot be conducted on acontinuous basis. (b) It is difficult, if not impossible, to exercisecontrol over the pore size and the porosity level of the resultingporous structure. (c) There is no flexibility in terms of varying theshape and size of the resulting reduced graphene oxide (RGO) material(e.g. it cannot be made into a film shape). (d) The method involves theuse of an ultra-low concentration of GO suspended in water (e.g. 2mg/mL=2 g/L=2 kg/kL). With the removal of non-carbon elements (up to50%), one can only produce less than 2 kg of graphene material (RGO) per1000-liter suspension. Furthermore, it is practically impossible tooperate a 1000-liter reactor that has to withstand the conditions of ahigh temperature and a high pressure. Clearly, this is not a scalableprocess for mass production of porous graphene structures.

The second method is based on a template-assisted catalytic CVD process,which involves CVD deposition of graphene on a sacrificial template(e.g. Ni foam). The graphene material conforms to the shape anddimensions of the Ni foam structure. The Ni foam is then etched awayusing an etching agent, leaving behind a monolith of graphene skeletonthat is essentially an open-cell foam. A useful reference for thismethod is given here: Zongping Chen, et al., “Three-dimensional flexibleand conductive interconnected graphene networks grown by chemical vapourdeposition,” Nature Materials, 10 (June 2011) 424-428. There are severalproblems associated with such a process: (a) the catalytic CVD isintrinsically a very slow, highly energy-intensive, and expensiveprocess; (b) the etching agent is typically a highly undesirablechemical and the resulting Ni-containing etching solution is a source ofpollution. It is very difficult and expensive to recover or recycle thedissolved Ni metal from the etchant solution. (c) It is challenging tomaintain the shape and dimensions of the graphene foam without damagingthe cell walls when the Ni foam is being etched away. The resultinggraphene foam is typically very brittle and fragile. (d) The transportof the CVD precursor gas (e.g. hydrocarbon) into the interior of a metalfoam can be difficult, resulting in a non-uniform structure, sincecertain spots inside the sacrificial metal foam may not be accessible tothe CVD precursor gas.

The third method of producing graphene foam also makes use of asacrificial material (e.g. colloidal polystyrene particles, PS) that iscoated with graphene oxide sheets using a self-assembly approach. Forinstance, Choi, et al. prepared chemically modified graphene (CMG) paperin two steps: fabrication of free-standing PS/CMG films by vacuumfiltration of a mixed aqueous colloidal suspension of CMG and PS (2.0 μmPS spheres), followed by removal of PS beads to generate 3D macro-pores.[B. G. Choi, et al., “3D Macroporous Graphene Frameworks forSupercapacitors with High Energy and Power Densities,” ACS Nano, 6(2012) 4020-4028.] Choi, et al. fabricated well-ordered free-standingPS/CMG paper by filtration, which began with separately preparing anegatively charged CMG colloidal and a positively charged PS suspension.A mixture of CMG colloidal and PS suspension was dispersed in solutionunder controlled pH (=2), where the two compounds had the same surfacecharges (zeta potential values of +13±2.4 mV for CMG and +68±5.6 mV forPS). When the pH was raised to 6, CMGs (zeta potential=−29±3.7 mV) andPS spheres (zeta potential=+51±2.5 mV) were assembled due to theelectrostatic interactions and hydrophobic characteristics between them,and these were subsequently integrated into PS/CMG composite paperthrough a filtering process. This method also has several shortcomings:(a) This method requires very tedious chemical treatments of bothgraphene oxide and PS particles. (b) The removal of PS by toluene alsoleads to weakened macro-porous structures. (c) Toluene is a highlyregulated chemical and must be treated with extreme caution. (d) Thepore sizes are typically excessively big (e.g. several μm), too big formany useful applications.

The above discussion clearly indicates that every prior art method orprocess for producing graphene foams has major deficiencies. Thus, it isan object of the present invention to provide a cost-effective processfor producing highly conductive, mechanically robust graphene-basedfoams (specifically, integral 3D graphene-carbon hybrid foam) in largequantities. This process does not involve the use of an environmentallyunfriendly chemical. This process enables the flexible design andcontrol of the porosity level and pore sizes.

It is another object of the present invention to provide a process forproducing graphene-carbon hybrid foams that exhibit a thermalconductivity, electrical conductivity, elastic modulus, and/or strengththat are comparable to or greater than those of the conventionalgraphite or carbon foams.

Yet another object of the present invention is to provide (a) a pristinegraphene-based hybrid foam that contains essentially all carbon only andpreferably have a meso-scaled pore size range (2-50 nm); and (b)non-pristine graphene foams (graphene fluoride, graphene chloride,nitrogenated graphene, etc.) that contains at least 0.001% by weight(typically from 0.01% to 25% by weight and most typically from 0.1% to20%) of non-carbon elements that can be used for a broad array ofapplications.

Another object of the present invention is to provide products (e.g.devices) that contain a graphene-carbon foam of the present inventionand methods of operating these products.

SUMMARY OF THE INVENTION

The present invention provides a method of producing an integral 3Dgraphene-carbon hybrid foam directly from particles of a graphiticmaterial and particles of a polymer. This method is stunningly simple.The method comprises:

-   -   (a) mixing multiple particles of a graphitic material and        multiple particles of a solid polymer carrier material to form a        mixture in an impacting chamber of an energy impacting        apparatus;    -   (b) operating this energy impacting apparatus with a frequency        and an intensity for a length of time sufficient for peeling off        graphene sheets from the graphitic material and transferring the        graphene sheets to surfaces of the solid polymer carrier        material particles to produce graphene-coated or        graphene-embedded polymer particles inside the impacting        chamber; (e.g. The impacting apparatus, when in operation,        imparts kinetic energy to polymer particles, which in turn        impinge upon graphite particle surfaces/edges and peel off        graphene sheets from the impacted graphite particles. These        peeled-off graphene sheets stick to surfaces of these polymer        particles. This is herein referred to as the “direct transfer”        process, meaning that graphene sheets are directly transferred        from graphite particles to surfaces of polymer particles without        being mediated by any third-party entity.)    -   (c) recovering the graphene-coated or graphene-embedded polymer        particles from the impacting chamber and consolidating the        graphene-coated or graphene-embedded polymer particles into a        desired shape of graphene-polymer hybrid structure (this        consolidating step can be as simple as a compacting step that        just packs graphene-coated or embedded particles into a desired        shape); and    -   (d) pyrolyzing this shape of graphene-polymer hybrid structure        to thermally convert the polymer into pores and carbon or        graphite that bonds the graphene sheets to form the integral 3D        graphene-carbon hybrid foam.

In certain alternative embodiments, a plurality of impacting balls ormedia are added to the impacting chamber of the energy impactingapparatus. These impacting balls, accelerated by the impactingapparatus, impact the surfaces/edges of graphite particles and peel offgraphene sheets therefrom. These graphene sheets are tentativelytransferred to surfaces of these impacting balls. Thesegraphene-supporting impacting balls subsequently impinge upon polymerparticles and transfer the supported graphene sheets to the surfaces ofthese polymer particles. This sequence of events is herein referred toas the “indirect transfer” process. In some embodiments of the indirecttransfer process, step (c) includes operating a magnet to separate theimpacting balls or media from the graphene-coated or graphene-embeddedpolymer particles.

The solid polymer material particles can include plastic or rubberbeads, pellets, spheres, wires, fibers, filaments, discs, ribbons, orrods, having a diameter or thickness from 10 nm to 10 mm. Preferably,the diameter or thickness is from 100 nm to 1 mm, and more preferablyfrom 200 nm to 200 μm. The solid polymer may be selected from solidparticles of a thermoplastic, thermoset resin, rubber, semi-penetratingnetwork polymer, penetrating network polymer, natural polymer, or acombination thereof. In an embodiment, the solid polymer is partiallyremoved by melting, etching, or dissolving in a solvent prior to step(d).

In certain embodiments, the graphitic material is selected from naturalgraphite, synthetic graphite, highly oriented pyrolytic graphite,graphite fiber, graphitic nano-fiber, graphite fluoride, oxidizedgraphite, chemically modified graphite, exfoliated graphite,recompressed exfoliated graphite, expanded graphite, meso-carbonmicro-bead, or a combination thereof. Preferably, the graphitic materialcontains a non-intercalated and non-oxidized graphitic material that hasnever been previously exposed to a chemical or oxidation treatment priorto the mixing step (a).

We have surprisingly observed that a broad array of impacting devicescan be used for practicing the instant invention. For instance, theenergy impacting apparatus can be a vibratory ball mill, planetary ballmill, high energy mill, basket mill, agitator ball mill, cryo ball mill,micro ball mill, tumbler ball mill, continuous ball mill, stirred ballmill, pressurized ball mill, freezer mill, vibratory sieve, bead mill,nano bead mill, ultrasonic homogenizer mill, centrifugal planetarymixer, vacuum ball mill, or resonant acoustic mixer.

For the formation of the carbon component of the resultinggraphene-carbon hybrid foam, one can choose polymer particles that havea high carbon yield or char yield (e.g. >30% by weight). The carbonyield is the weight percentage of a polymer structure that is convertedby heat to a solid carbon phase, instead of becoming part of a volatilegas. The high carbon-yield polymer may be selected from phenolic resin,poly furfuryl alcohol, polyacrylonitrile, polyimide, polyamide,polyoxadiazole, polybenzoxazole, polybenzobisoxazole, polythiazole,polybenzothiazole, polybenzobisthiazole, poly(p-phenylene vinylene),polybenzimidazole, polybenzobisimidazole, a copolymer thereof, a polymerblend thereof, or a combination thereof.

If a lower carbon content (higher graphene proportion) is desired in thegraphene-carbon hybrid foam, the polymer can contain a low carbon-yieldpolymer selected from polyethylene, polypropylene, polybutylene,polyvinyl chloride, polycarbonate, acrylonitrile-butadiene (ABS),polyester, polyvinyl alcohol, polyvinylidiene fluoride (PVDF),polytetrafluoroethylene (PTFE), polyphenylene oxide (PPO), poly methylmethacrylate (PMMA), a copolymer thereof, a polymer blend thereof, or acombination thereof.

It may be noted that these polymers (both high and low carbon yields),when heated at a temperature of 300-2,500° C., are converted into acarbon material, which is preferentially nucleated near graphene sheetedges. Such a carbon material serves to bridge the gaps between graphenesheets, forming interconnected electron-conducting pathways. In otherwords, the resulting graphene-carbon hybrid foam is composed of integral3D network of carbon-bonded graphene sheets, allowing continuoustransport of electrons and phonons (quantized lattice vibrations)between graphene sheets or domains without interruptions. When furtherheated at a temperature higher than 2,500° C., the graphene-bondingcarbon phase can get graphitized provided that the carbon phase is “softcarbon” or graphitizable. In such a situation, both the electricconductivity and thermal conductivity are further increased.

Thus, in certain embodiments, the step of pyrolyzing includescarbonizing the polymer at a temperature from 200° C. to 2,500° C. toobtain carbon-bonded graphene sheets. Optionally, the carbon-bondedgraphene sheets can be subsequently graphitized at a temperature from2,500° C. to 3,200° C. to obtain graphite-bonded graphene sheets.

It may be noted that pyrolyzation of a polymer tends to lead to theformation of pores in the resulting polymeric carbon phase due to theevolution of those volatile gas molecules such as CO₂ and H₂O. However,such pores also have a high tendency to get collapsed if the polymer isnot constrained when being carbonized. We have surprisingly discoveredthat the graphene sheets wrapped around a polymer particle are capableof constraining the carbon pore walls from being shrunk and collapsed,while some carbon species also permeate to the gaps between graphenesheets where these species bond the graphene sheets together. The poresizes and pore volume (porosity level) of the resulting 3D integralgraphene foam depend upon the starting polymer size and the carbon yieldof the polymer and, to a lesser extent, on the pyrolyzation temperature.

In certain preferred embodiments, the consolidating step includescompacting a mass of these graphene-coated polymer particles into adesired shape. For instance, by squeezing and compressing the mass ofgraphene-coated particles into a mold cavity one can readily form acompact green body. One can rapidly heat and melt the polymer, slightlycompress the green body to slightly fuse the polymer particles togetherby heat, and rapidly cool to solidify the body. This consolidated bodyis then subjected to a pyrolysis treatment (polymer carbonization and,optionally, graphitization).

In some alternative embodiments, the consolidating step includes meltingthe polymer particles to form a polymer melt mixture with graphenesheets dispersed therein, forming the polymer melt mixture into adesired shape and solidifying the shape into a graphene-polymercomposite structure. Such shape can be a rod, film (thin or thick film,wide or narrow, single sheets or in a roll), fiber (short filament orcontinuous long filament), plate, ingot, any regular shape or odd shape.This graphene-polymer composite shape is then pyrolyzed

Alternatively, the consolidating step may include dissolving the polymerparticles in a solvent to form a polymer solution mixture with graphenesheets dispersed therein, forming the polymer solution mixture into adesired shape, and removing the solvent to solidify the shape into thegraphene-polymer composite structure. This composite structure is thenpyrolyzed to form a porous structure.

The consolidating step may include melting the polymer particles to forma polymer melt mixture with graphene sheets dispersed therein andextruding the mixture into a rod form or sheet form, spinning themixture into a fiber form, spraying the mixture into a powder form, orcasting the mixture into an ingot form.

In some embodiments, the consolidating step includes dissolving thepolymer particles in a solvent to form a polymer solution mixture withgraphene sheets dispersed therein and extruding the solution mixtureinto a rod form or sheet form, spinning the solution mixture into afiber form, spraying the solution mixture into a powder form, or castingthe solution mixture into an ingot form, and removing the solvent.

In a specific embodiment, the polymer solution mixture is sprayed tocreate a graphene-polymer composite coating or film, which is thenpyrolyzed (carbonized or carbonized and graphitized).

Preferably, the consolidating step may include compacting thegraphene-coated polymer particles in a porous green compact havingmacroscopic pores and then infiltrate or impregnate the pores with anadditional carbon source material selected from a petroleum pitch, coaltar pitch, an aromatic organic material (e.g. naphthalene or otherderivatives of a pitch), a monomer, an organic polymer, or a combinationthereof. The organic polymer may contain a high carbon-yield polymerselected from phenolic resin, poly furfuryl alcohol, polyacrylonitrile,polyimide, polyamide, polyoxadiazole, polybenzoxazole,polybenzobisoxazole, polythiazole, polybenzothiazole,polybenzobisthiazole, poly(p-phenylene vinylene), polybenzimidazole,polybenzobisimidazole, a copolymer thereof, a polymer blend thereof, ora combination thereof. When the infiltrated green compact ofgraphene-coated polymer particles is subjected to pyrolyzation, thesespecies become additional sources of carbon, if a higher amount ofcarbon in the hybrid foam is desired.

The present invention also provides an integral 3D graphene-carbonhybrid foam composed of multiple pores and pore walls, wherein the porewalls contain single-layer or few-layer graphene sheets chemicallybonded by a carbon material having a carbon material-to-graphene weightratio from 1/200 to 1/2, wherein the few-layer graphene sheets have 2-10layers of stacked graphene planes having an inter-plane spacing d₀₀₂from 0.3354 nm to 0.36 nm as measured by X-ray diffraction and thesingle-layer or few-layer graphene sheets contain a pristine graphenematerial having essentially zero % of non-carbon elements, or anon-pristine graphene material having 0.001% to 35% by weight(preferably 0.01% to 25%) of non-carbon elements wherein thenon-pristine graphene is selected from graphene oxide, reduced grapheneoxide, graphene fluoride, graphene chloride, graphene bromide, grapheneiodide, hydrogenated graphene, nitrogenated graphene, chemicallyfunctionalized graphene, or a combination thereof. A plurality ofsingle-layer or few layer graphene embracing the underlying polymerparticles can overlap with one another to form a stack of graphenesheets. The stack can have a thickness greater than 5 nm and, in somecases, greater than 10 nm or even greater than 100 nm.

The integral 3D graphene-carbon hybrid foam typically has a density from0.001 to 1.7 g/cm³, and a specific surface area from 50 to 3,000 m²/g.In a preferred embodiment, the pore walls contain stacked grapheneplanes having an inter-planar spacing d₀₀₂ from 0.3354 nm to 0.40 nm asmeasured by X-ray diffraction.

For oil recovery applications (e.g. separating oil from water), thegraphene-carbon preferably has an oxygen content from 1% to 25% byweight (more preferably 1-15% and most preferably 1-10%). This can beachieved if the starting material is oxidized graphite, or if thecarbonization treatment is conducted in a lightly oxidizing environmentat a temperature of 300-1,500° C. (preferably no greater than 1,000° C.)and no subsequent graphitization is conducted. We have surprisinglydiscovered that a highly porous graphene-carbon foam of this nature iscapable of absorbing oil from an oil-water mixture up to 500% of its ownweight.

For thermal management applications, the graphene-carbon hybrid foam ispreferably made by subjecting the carbon-bonded graphene sheets (aftercarbonization) to a graphitization treatment under a compressive stress.This facilitates orientation and re-organization (merging, growth, etc.)of graphene sheets or graphene domains. As a result, the graphene-carbonfoam sheet or film exhibits a thermal conductivity of at least 200 W/mKper unit of specific gravity, and/or an electrical conductivity no lessthan 2,000 S/cm per unit of specific gravity.

In an embodiment, the pore walls contain pristine graphene and the 3Dsolid graphene-carbon foam has a density from 0.001 to 1.7 g/cm³ or anaverage pore size from 2 nm to 50 nm. In an embodiment, the pore wallscontain a non-pristine graphene material selected from the groupconsisting of graphene oxide, reduced graphene oxide, graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene, andcombinations thereof, and wherein the solid graphene foam contains acontent of non-carbon elements in the range of 0.01% to 20% by weight.In other words, the non-carbon elements can include an element selectedfrom oxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, orboron. In a specific embodiment, the pore walls contain graphenefluoride and the solid graphene foam contains a fluorine content from0.01% to 20% by weight. In another embodiment, the pore walls containgraphene oxide and said solid graphene foam contains an oxygen contentfrom 0.01% to 20% by weight. In an embodiment, the solid graphene-carbonhybrid foam has a specific surface area from 200 to 2,000 m²/g or adensity from 0.01 to 1.5 g/cm³.

It may be noted that there are no limitations on the shape or dimensionsof the presently invented graphene-carbon hybrid foam. In a preferredembodiment, the solid graphene-carbon hybrid foam is made into acontinuous-length roll sheet form (a roll of a continuous foam sheet)having a thickness no less than 100 nm and no greater than 10 cm and alength of at least 1 meter long, preferably at least 2 meters, furtherpreferably at least 10 meters, and most preferably at least 100 meters.This sheet roll is produced by a roll-to-roll process. There has been noprior art graphene-based foam that is made into a sheet roll form. Ithas not been previously found or suggested possible to have aroll-to-roll process for producing a continuous length of graphene foam,either pristine or non-pristine based.

For thermal management or electrical conductivity-based applications,the graphene-carbon foam preferably has an oxygen content or non-carboncontent less than 1% by weight, and the pore walls have stacked grapheneplanes having an inter-graphene spacing less than 0.35 nm, a thermalconductivity of at least 250 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 2,500 S/cm per unit of specificgravity.

In a further preferred embodiment, the graphene-carbon hybrid foam hasan oxygen content or non-carbon content less than 0.01% by weight andsaid pore walls contain stacked graphene planes having an inter-graphenespacing less than 0.34 nm, a thermal conductivity of at least 300 W/mKper unit of specific gravity, and/or an electrical conductivity no lessthan 3,000 S/cm per unit of specific gravity.

In yet another preferred embodiment, the graphene-carbon hybrid foam hasan oxygen content or non-carbon content no greater than 0.01% by weightand said pore walls contain stacked graphene planes having aninter-graphene spacing less than 0.336 nm, a mosaic spread value nogreater than 0.7, a thermal conductivity of at least 350 W/mK per unitof specific gravity, and/or an electrical conductivity no less than3,500 S/cm per unit of specific gravity.

In still another preferred embodiment, the graphene foam has pore wallscontaining stacked graphene planes having an inter-graphene spacing lessthan 0.336 nm, a mosaic spread value no greater than 0.4, a thermalconductivity greater than 400 W/mK per unit of specific gravity, and/oran electrical conductivity greater than 4,000 S/cm per unit of specificgravity.

In a preferred embodiment, the pore walls contain stacked grapheneplanes having an inter-graphene spacing less than 0.337 nm and a mosaicspread value less than 1.0. In a preferred embodiment, the graphene foamexhibits a degree of graphitization no less than 80% (preferably no lessthan 90%) and/or a mosaic spread value less than 0.4. In a preferredembodiment, the pore walls contain a 3D network of interconnectedgraphene planes.

In a preferred embodiment, the solid graphene-carbon hybrid foamcontains meso-scaled pores having a pore size from 2 nm to 50 nm. Thesolid graphene foam can also be made to contain micron-scaled pores(1-500 μm).

The present invention also provides an oil-removing or oil-separatingdevice, which contains the presently invented 3D graphene-carbon hybridfoam as an oil-absorbing element. Also provided is a solvent-removing orsolvent-separating device containing the 3D graphene-carbon hybrid foamas a solvent-absorbing element.

The invention also provides a method to separate oil from an oil-watermixture (e.g. oil-spilled water or waste water from oil sand). Themethod comprises the steps of (a) providing an oil-absorbing elementcomprising an integral graphene-carbon hybrid foam; (b) contacting anoil-water mixture with the element, which absorbs the oil from themixture; and (c) retreating the element from the mixture and extractingthe oil from the element. Preferably, the method comprises a furtherstep of (d) reusing the element.

Additionally, the invention provides a method to separate an organicsolvent from a solvent-water mixture or from a multiple-solvent mixture.The method comprises the steps of (a) providing an organicsolvent-absorbing element comprising an integral graphene-carbon hybridfoam; (b) bringing the element in contact with an organic solvent-watermixture or a multiple-solvent mixture containing a first solvent and atleast a second solvent; (c) allowing this element to absorb the organicsolvent from the mixture or absorb the first solvent from the at leastsecond solvent; and (d) retreating the element from the mixture andextracting the organic solvent or first solvent from the element.Preferably, the method contains an additional step (e) of reusing theelement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A flow chart showing the most commonly used prior art process ofproducing highly oxidized NGPs that entails tedious chemicaloxidation/intercalation, rinsing, and high-temperature exfoliationprocedures.

FIG. 2(A) A flow chart showing the presently invented process forproducing integral 3D graphene-carbon hybrid foam.

FIG. 2(B) Schematic of the heat-induced conversion of polymer intocarbon, which bonds graphene sheets together to form a 3Dgraphene-carbon hybrid foam. The compacted structure of graphene-coatedpolymer particles is converted into a highly porous structure.

FIG. 3(A) An SEM image of an internal structure of a 3D graphene-carbonhybrid foam.

FIG. 3(B) An SEM image of an internal structure of another 3Dgraphene-carbon hybrid foam

FIG. 4(A) Thermal conductivity values vs. specific gravity of a 3Dintegral graphene-carbon foam produced by the presently inventedprocess, a meso-phase pitch-derived graphite foam, and a Nifoam-template assisted CVD graphene foam.

FIG. 4(B) Thermal conductivity values of 3D graphene-carbon foam and thehydrothermally reduced GO graphene foam.

FIG. 5 Thermal conductivity values of 3D graphene-carbon hybrid foam andpristine graphene foam (prepared by casting with a blowing agent andthen heat treating) plotted as a function of the final (maximum) heattreatment temperature.

FIG. 6 Electrical conductivity values of 3D graphene-carbon foam and thehydrothermally reduced GO graphene foam.

FIG. 7 The amount of oil absorbed per gram of integral 3Dgraphene-carbon hybrid foam, plotted as a function of the oxygen contentin the foam having a porosity level of approximately 98% (oil separationfrom oil-water mixture).

FIG. 8 The amount of oil absorbed per gram of integral 3Dgraphene-carbon hybrid foam, plotted as a function of the porosity level(given the same oxygen content).

FIG. 9 The amount of chloroform absorbed out of a chloroform-watermixture, plotted as a function of the degree of fluorination.

FIG. 10 Schematic of heat sink structures (2 examples).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method of producing an integral 3Dgraphene-carbon hybrid foam directly from particles of a graphiticmaterial and particles of a polymer.

As schematically illustrated in FIG. 2(A), the method begins with mixingmultiple particles of a graphitic material and multiple particles of asolid polymer carrier material to form a mixture, which is enclosed inan impacting chamber of an energy impacting apparatus (e.g. a vibratoryball mill, planetary ball mill, high energy mill, basket mill, agitatorball mill, cryo ball mill, micro ball mill, tumbler ball mill,continuous ball mill, stirred ball mill, pressurized ball mill, freezermill, vibratory sieve, bead mill, nano bead mill, ultrasonic homogenizermill, centrifugal planetary mixer, vacuum ball mill, or resonantacoustic mixer). When in operation, this energy impacting device impartskinetic energy to the solid particles contained therein, allowingpolymer particles to impinge upon graphite particles with high intensityand high frequency.

In typical operational conditions, such impacting events result inpeeling off of graphene sheets from the graphitic material andtransferring the graphene sheets to surfaces of the solid polymercarrier particles. These graphene sheets wrap around polymer particlesto form graphene-coated or graphene-embedded polymer particles insidethe impacting chamber. This is herein referred to as the “directtransfer” process, meaning that graphene sheets are directly transferredfrom graphite particles to surfaces of polymer particles without beingmediated by any third-party entities.

Alternatively, a plurality of impacting balls or media can be added tothe impacting chamber of the energy impacting apparatus. These impactingballs, accelerated by the impacting apparatus, impinge upon thesurfaces/edges of graphite particles with a high kinetic energy at afavorable angle to peel off graphene sheets from graphite particles.These graphene sheets are tentatively transferred to surfaces of theseimpacting balls. These graphene-supporting impacting balls subsequentlycollide with polymer particles and transfer the supported graphenesheets to the surfaces of these polymer particles. This sequence ofevents is herein referred to as the “indirect transfer” process. Theseevents occur in very high frequency and, hence, most of the polymerparticles are covered by graphene sheets typically in less than onehour. In some embodiments of the indirect transfer process, step (c)includes operating a magnet to separate the impacting balls or mediafrom the graphene-coated or graphene-embedded polymer particles.

The method then includes recovering the graphene-coated orgraphene-embedded polymer particles from the impacting chamber andconsolidating the graphene-coated or graphene-embedded polymer particlesinto a desired shape of graphene-polymer composite structure. Thisconsolidating step can be as simple as a compacting step that justmechanically packs graphene-coated or embedded particles into a desiredshape. Alternatively, this consolidating step can entail melting thepolymer particles to form a polymer matrix with graphene sheetsdispersed therein. Such a graphene-polymer structure can be in anypractical shape or dimensions (fiber, rod, plate, cylinder, or anyregular shape or odd shape).

The graphene-polymer compact or composite structure is then pyrolyzed tothermally convert the polymer into carbon or graphite that bonds thegraphene sheets to form the integral 3D graphene-carbon hybrid foam.

For the formation of the carbon component of the resultinggraphene-carbon hybrid foam, one can choose polymer particles that havea high carbon yield or char yield (e.g. >30% by weight of a polymerbeing converted to a solid carbon phase; instead of becoming part of avolatile gas). The high carbon-yield polymer may be selected fromphenolic resin, poly furfuryl alcohol, polyacrylonitrile, polyimide,polyamide, polyoxadiazole, polybenzoxazole, polybenzobisoxazole,polythiazole, polybenzothiazole, polybenzobisthiazole, poly(p-phenylenevinylene), polybenzimidazole, polybenzobisimidazole, a copolymerthereof, a polymer blend thereof, or a combination thereof. Whenpyrolyzed, particles of these polymers become porous, as illustrated inthe bottom portion of FIG. 2(B).

If a lower carbon content (higher graphene proportion relative to carbonproportion) and lower foam density are desired in the graphene-carbonhybrid foam, the polymer can contain a low carbon-yield polymer selectedfrom polyethylene, polypropylene, polybutylene, polyvinyl chloride,polycarbonate, acrylonitrile-butadiene (ABS), polyester, polyvinylalcohol, poly vinyldiene fluoride (PVDF), polytetrafluoroethylene(PTFE), polyphenylene oxide (PPO), poly methyl methacrylate (PMMA), acopolymer thereof, a polymer blend thereof, or a combination thereof.When pyrolyzed, particles of these polymers become porous, asillustrated in the middle portion of FIG. 2(B).

These polymers (both high and low carbon yields), when heated at atemperature of 300-2,500° C., are converted into a carbon material,which is preferentially nucleated near graphene sheet edges. Such acarbon material naturally bridges the gaps between graphene sheets,forming interconnected electron-conducting pathways. In actuality, theresulting graphene-carbon hybrid foam is composed of integral 3D networkof carbon-bonded graphene sheets, enabling continuous transport ofelectrons and phonons (quantized lattice vibrations) between graphenesheets or domains without interruptions. When further heated at atemperature higher than 2,500° C., the carbon phase can get graphitizedto further increase both the electric conductivity and thermalconductivity. The amount of non-carbon elements is also decreased totypically below 1% by weight if the graphitization time exceeds 1 hour.

It may be noted that an organic polymer typically contains a significantamount of non-carbon elements, which can be reduced or eliminated viaheat treatments. As such, pyrolyzation of a polymer causes the formationand evolution of volatile gas molecules, such as CO₂ and H₂O, which leadto the formation of pores in the resulting polymeric carbon phase.However, such pores also have a high tendency to get collapsed if thepolymer is not constrained when being carbonized (the carbon structurecan shrink while non-carbon elements are being released). We havesurprising discovered that the graphene sheets wrapped around a polymerparticle are capable of constraining the carbon pore walls from beingcollapsed. In the meantime, some carbon species also permeate to thegaps between graphene sheets where these species bond the graphenesheets together. The pore sizes and pore volume (porosity level) of theresulting 3D integral graphene foam mainly depend upon the startingpolymer size and the carbon yield of the polymer.

The graphitic material, as a source of graphene sheets, may be selectedfrom natural graphite, synthetic graphite, highly oriented pyrolyticgraphite, graphite fiber, graphitic nano-fiber, graphite fluoride,oxidized graphite, chemically modified graphite, exfoliated graphite,recompressed exfoliated graphite, expanded graphite, meso-carbonmicro-bead, or a combination thereof. In this regard, there are severaladditional surprising elements associated with the presently inventedmethod:

-   -   (1) Graphene sheets can be peeled off from natural graphite by        using polymer particles alone, without utilizing the heavier and        harder impacting balls (such as zirconium dioxide or steel balls        commonly used in a ball mill, for instance). The peeled-off        graphene sheets are directly transferred to polymer particle        surfaces and are firmly wrapped around the polymer particles.    -   (2) It is also surprising that impacting polymer particles are        capable of peeling off graphene sheets from artificial graphite,        such as meso-carbon micro-beads (MCMBs), which are known to have        a skin layer of amorphous carbon.    -   (3) With the assistance of harder impacting balls, the        graphene-like planes of carbon atoms constituting the internal        structure of a carbon or graphite fiber can also be peeled off        and transferred to the polymer particle surfaces. This has never        been taught or suggested in prior art.    -   (4) The present invention provides a strikingly simple, fast,        scalable, environmentally benign, and cost-effective process        that avoids essentially all of the drawbacks associated with        prior art processes of producing graphene sheets. The graphene        sheets are immediately transferred to and wrapped around the        polymer particles, which are then readily converted to integral        3D graphene-carbon hybrid foam.

It may be noted that a certain desired degree of hydrophilicity can beimparted to the pore walls of the graphene-carbon hybrid foam if thestarting graphite is intentionally oxidized to some degree (e.g. tocontain 2-15% by weight of oxygen). Alternatively, one can attachoxygen-containing functional groups to the carbon phase if thecarbonization treatment is allowed to occur in a slightly oxidizingenvironment. These features enable separation of oil from water byselectively absorbing oil from an oil-water mixture. In other words,such a graphene-carbon hybrid foam material is capable of recovering oilfrom water, helping to clean up oil-spilled river, lake, or ocean. Theoil absorption capacity is typically from 50% to 500% of the foam's ownweight. This is a wonderfully useful material for environmentalprotection purposes.

If a high electrical or thermal conductivity is desired, the graphiticmaterial may be selected from a non-intercalated and non-oxidizedgraphitic material that has never been previously exposed to a chemicalor oxidation treatment prior to being placed into the impacting chamber.Alternatively or additionally, the graphene-carbon foam can be subjectedto graphitization treatment at a temperature higher than 2,500° C. Theresulting material is particularly useful for thermal managementapplications (e.g. for use to make a finned heat sink, a heat exchanger,or a heat spreader.

It may be noted that the graphene-carbon foam may be subjected tocompression during and/or after the graphitization treatment. Thisoperation enables us to adjust the graphene sheet orientation and thedegree of porosity.

X-ray diffraction patterns were obtained with an X-ray diffractometerequipped with CuKcv radiation. The shift and broadening of diffractionpeaks were calibrated using a silicon powder standard. The degree ofgraphitization, g, was calculated from the X-ray pattern using theMering's Eq, d₀₀₂=0.3354 g+0.344 (1−g), where d₀₀₂ is the interlayerspacing of graphite or graphene crystal in nm. This equation is validonly when d₀₀₂ is equal or less than approximately 0.3440 nm. Thegraphene foam walls having a d₀₀₂ higher than 0.3440 nm reflects thepresence of oxygen- or fluorine-containing functional groups (such as—F, —OH, >O, and —COOH on graphene molecular plane surfaces or edges)that act as a spacer to increase the inter-graphene spacing.

Another structural index that can be used to characterize the degree ofordering of the stacked and bonded graphene planes in the foam walls ofgraphene and conventional graphite crystals is the “mosaic spread,”which is expressed by the full width at half maximum of a rocking curve(X-ray diffraction intensity) of the (002) or (004) reflection. Thisdegree of ordering characterizes the graphite or graphene crystal size(or grain size), amounts of grain boundaries and other defects, and thedegree of preferred grain orientation. A nearly perfect single crystalof graphite is characterized by having a mosaic spread value of 0.2-0.4.Most of our graphene walls have a mosaic spread value in this range of0.2-0.4 (if produced with a heat treatment temperature (HTT) no lessthan 2,500° C.). However, some values are in the range of 0.4-0.7 if theHTT is between 1,500 and 2,500° C., and in the range of 0.7-1.0 if theHTT is between 300 and 1,500° C.

In-depth studies using a combination of SEM, TEM, selected areadiffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIRindicate that the graphene foam walls are composed of several hugegraphene planes (with length/width typically >>20 nm, moretypically >>100 nm, often >>1 μm, and, in many cases, >>10 μm, oreven >>100 μm). These giant graphene planes are stacked and bonded alongthe thickness direction (crystallographic c-axis direction) oftenthrough not just the van der Waals forces (as in conventional graphitecrystallites), but also covalent bonds, if the final heat treatmenttemperature is lower than 2,500° C. In these cases, wishing not to belimited by theory, but Raman and FTIR spectroscopy studies appear toindicate the co-existence of sp² (dominating) and sp³ (weak butexisting) electronic configurations, not just the conventional sp² ingraphite.

The integral 3D graphene-carbon hybrid foam is composed of multiplepores and pore walls, wherein the pore walls contain single-layer orfew-layer graphene sheets chemically bonded by a carbon material havinga carbon material-to-graphene weight ratio from 1/100 to 1/2, whereinthe few-layer graphene sheets have 2-10 layers of stacked grapheneplanes having an inter-plane spacing d₀₀₂ from 0.3354 nm to 0.36 nm asmeasured by X-ray diffraction and the single-layer or few-layer graphenesheets contain a pristine graphene material having essentially zero % ofnon-carbon elements, or a non-pristine graphene material having 0.01% to25% by weight of non-carbon elements (more typically <15%) wherein thenon-pristine graphene is selected from graphene oxide, reduced grapheneoxide, graphene fluoride, graphene chloride, graphene bromide, grapheneiodide, hydrogenated graphene, nitrogenated graphene, chemicallyfunctionalized graphene, or a combination thereof. A plurality ofsingle-layer or few layer graphene embracing the underlying polymerparticles can overlap with one another to form a stack of graphenesheets. The stack can have a thickness greater than 5 nm and, in somecases, greater than 10 nm or even greater than 100 nm.

The integral 3D graphene-carbon hybrid foam typically has a density from0.001 to 1.7 g/cm³, a specific surface area from 50 to 3,000 m²/g, athermal conductivity of at least 200 W/mK per unit of specific gravity,and/or an electrical conductivity no less than 2,000 S/cm per unit ofspecific gravity. In a preferred embodiment, the pore walls containstacked graphene planes having an inter-planar spacing d₀₀₂ from 0.3354nm to 0.40 nm as measured by X-ray diffraction.

Many of the graphene sheets can be merged edge to edge through covalentbonds with one another, into an integrated graphene entity. The gapsbetween the free ends of those unmerged sheets or shorter merged sheetsare bonded by the carbon phase converted from a polymer. Due to theseunique chemical composition (including oxygen or fluorine content,etc.), morphology, crystal structure (including inter-graphene spacing),and structural features (e.g. degree of orientations, few defects,chemical bonding and no gap between graphene sheets, and substantiallyno interruptions along graphene plane directions), the graphene-carbonhybrid foam has a unique combination of outstanding thermalconductivity, electrical conductivity, mechanical strength, andstiffness (elastic modulus).

Thermal Management Applications

The aforementioned features and characteristics make the integral 3Dgraphene-carbon hybrid foam an ideal element for a broad array ofengineering and biomedical applications. For instance, for thermalmanagement purposes alone, the graphene-carbon foam can be used in thefollowing applications:

-   -   a) The graphene-carbon hybrid foam, being compressible and of        high thermal conductivity, is ideally suited for use as a        thermal interface material (TIM) that can be implemented between        a heat source and a heat spreader or between a heat source and a        heat sink.    -   b) The hybrid foam can be used as a heat spreader per se due to        its high thermal conductivity.    -   c) The hybrid foam can be used as a heat sink or heat        dissipating material due to his high heat-spreading capability        (high thermal conductivity) and high heat-dissipating capability        (large number of surface pores inducing massive air-convection        micro or nano channels).    -   d) The light weight (low density adjustable between 0.001 and        1.8 g/cm³), high thermal conductivity per unit specific gravity        or per unit of physical density, and high structural integrity        (graphene sheets being bonded by carbon) make this hybrid foam        an ideal material for a durable heat exchanger.

The graphene-carbon hybrid foam-based thermal management or heatdissipating devices include a heat exchanger, a heat sink (e.g. finnedheat sink), a heat pipe, high-conductivity insert, thin or thickconductive plate (between a heat sink and a heat source), thermalinterface medium (or thermal interface material, TIM), thermoelectric orPeltier cooling plate, etc.

A heat exchanger is a device used to transfer heat between one or morefluids; e.g. a gas and a liquid separately flowing in differentchannels. The fluids are typically separated by a solid wall to preventmixing. The presently invented graphene-carbon hybrid foam material isan ideal material for such a wall provided the foam is not a totallyopen-cell foam that allows for mixing of fluids. The presently inventedmethod enables production of both open-cell and closed-cell foamstructures. The high surface pore areas enable dramatically fasterexchange of heats between the two or multiple fluids.

Heat exchangers are widely used in refrigeration systems, airconditioning units, heaters, power stations, chemical plants,petrochemical plants, petroleum refineries, natural-gas processing, andsewage treatment. A well-known example of a heat exchanger is found inan internal combustion engine in which a circulating engine coolantflows through radiator coils while air flows past the coils, which coolsthe coolant and heats the incoming air. The solid walls (e.g. thatconstitute the radiator coils) are normally made of a high thermalconductivity material, such as Cu and Al. The presently inventedgraphene foam having either a higher thermal conductivity or higherspecific surface area is a superior alternative to Cu and Al, forinstance.

There are many types of heat exchangers that are commercially available:shell and tube heat exchanger, plate heat exchangers, plate and shellheat exchanger, adiabatic wheel heat exchanger, plate fin heatexchanger, pillow plate heat exchanger, fluid heat exchangers, wasteheat recovery units, dynamic scraped surface heat exchanger,phase-change heat exchangers, direct contact heat exchangers, andmicrochannel heat exchangers. Every one of these types of heatexchangers can take advantage of the exceptional high thermalconductivity and specific surface area of the presently invented foammaterial.

The presently invented solid graphene foam can also be used in a heatsink. Heat sinks are widely used in electronic devices for heatdissipation purposes. The central processing unit (CPU) and battery in aportable microelectronic device (such as a notebook computer, tablet,and smart phone) are well-known heat sources. Typically, a metal orgraphite object (e.g. Cu foil or graphite foil) is brought into contactwith the hot surface and this object helps to spread the heat to anexternal surface or outside air (primarily by conduction and convectionand to a lesser extent by radiation). In most cases, a thin thermalinterface material (TIM) mediates between the hot surface of the heatsource and a heat spreader or a heat-spreading surface of a heat sink.

A heat sink usually consists of a high-conductivity material structurewith one or more flat surfaces to ensure good thermal contact with thecomponents to be cooled, and an array of comb or fin like protrusions toincrease the surface contact with the air, and thus the rate of heatdissipation. A heat sink may be used in conjunction with a fan toincrease the rate of airflow over the heat sink. A heat sink can havemultiple fins (extended or protruded surfaces) to improve heat transfer.In electronic devices with limited amount of space, theshape/arrangement of fins must be optimized such that the heat transferdensity is maximized. Alternatively or additionally, cavities (invertedfins) may be embedded in the regions formed between adjacent fins. Thesecavities are effective in extracting heat from a variety of heatgenerating bodies to a heat sink.

Typically, an integrated heat sink comprises a heat collection member(core or base) and at least one heat dissipation member (e.g. a fin ormultiple fins) integral to the heat collection member (base) to form afumed heat sink. The fins and the core are naturally connected orintegrated together into a unified body without using an externallyapplied adhesive or mechanical fastening means to connect the fins tothe core. The heat collection base has a surface in thermal contact witha heat source (e.g. a LED), collects heat from this heat source, anddissipates heat through the fins into the air.

As illustrative examples, FIG. 10 provides a schematic of two heatsinks: 300 and 302. The first one contains a heat collection member (orbase member) 304 and multiple fins or heat dissipation members (e.g. fin306) connected to the base member 304. The base member 304 is shown tohave a heat collection surface 314 intended to be in thermal contactwith a heat source. The heat dissipation member or fin 306 is shown tohave at least a heat dissipation surface 320.

A particularly useful embodiment is an integrated radial heat sink 302comprising a radial finned heat sink assembly that comprises: (a) a base308 comprising a heat collection surface 318; and (b) a plurality ofspaced parallel planar fin members (e.g. 310, 312 as two examples)supported by or integral with the base 308, wherein the planar finmembers (e.g. 310) comprise the at least one heat dissipation surface322. Multiple parallel planar fin members are preferably equally spaced.

The presently invented graphene-carbon hybrid foam, being highly elasticand resilient, is itself a good thermal interface material and a highlyeffective heat spreading element as well. In addition, thishigh-conductivity foam can also be used as an inserts for electroniccooling and for enhancing the heat removal from small chips to a heatsink. Because the space occupied by high conductivity materials is amajor concern, it is a more efficient design to make use of highconductivity pathways that can be embedded into a heat generating body.The elastic and highly conducting solid graphene foam herein disclosedmeets these requirements perfectly.

The high elasticity and high thermal conductivity make the presentlyinvented solid graphene-carbon hybrid foam a good conductive thick plateto be placed as a heat transfer interface between a heat source and acold flowing fluid (or any other heat sink) to improve the coolingperformance. In such arrangement, the heat source is cooled under thethick graphene foam plate instead of being cooled in direct contact withthe cooling fluid. The thick plate of graphene foam can significantlyimprove the heat transfer between the heat source and the cooling fluidby way of conducting the heat current in an optimal manner. Noadditional pumping power and no extra heat transfer surface area arerequired.

The solid graphene foam is also an outstanding material to construct aheat pipe. A heat pipe is a heat transfer device that uses evaporationand condensation of a two-phase working fluid or coolant to transportlarge quantities of heat with a very small difference in temperaturebetween the hot and cold interfaces. A conventional heat pipe consistsof sealed hollow tube made of a thermally conductive metal such as Cu orAl, and a wick to return the working fluid from the evaporator to thecondenser. The pipe contains both saturated liquid and vapor of aworking fluid (such as water, methanol or ammonia), all other gasesbeing excluded. However, both Cu and Al are prone to oxidation orcorrosion and, hence, their performance degrades relatively fast overtime. In contrast, the solid graphene foam is chemically inert and doesnot have these oxidation or corrosion issues. The heat pipe forelectronics thermal management can have a solid graphene foam envelopeand wick, with water as the working fluid. Graphene/methanol may be usedif the heat pipe needs to operate below the freezing point of water, andgraphene/ammonia heat pipes may be used for electronics cooling inspace.

Peltier cooling plates operate on the Peltier effect to create a heatflux between the junction of two different conductors of electricity byapplying an electric current. This effect is commonly used for coolingelectronic components and small instruments. In practice, many suchjunctions may be arranged in series to increase the effect to the amountof heating or cooling required. The solid graphene foam may be used toimprove the heat transfer efficiency.

Filtration and Fluid Absorption Applications

The solid graphene foam can be made to contain microscopic pores (<2 nm)or meso-scaled pores having a pore size from 2 nm to 50 nm. The solidgraphene-carbon hybrid foam can also be made to contain micron-scaledpores (1-500 μm). Based on well-controlled pore size alone, the instantgraphene-carbon foam can be an exceptional filter material for air orwater filtration.

Further, the graphene pore wall chemistry and carbon phase chemistry canbe independently controlled to impart different amounts and/or types offunctional groups to either or both of the graphene sheets and thecarbon binder phase (e.g. as reflected by the percentage of O, F, N, H,etc. in the foam). In other words, the concurrent or independent controlof both pore sizes and chemical functional groups at different sites ofthe internal structure provide unprecedented flexibility or highestdegree of freedom in designing and making graphene-carbon hybrid foamsthat exhibit many unexpected properties, synergistic effects, and someunique combination of properties that are normally considered mutuallyexclusive (e.g. some part of the structure is hydrophobic and other parthydrophilic; or the foam structure is both hydrophobic and oleophilic).A surface or a material is said to be hydrophobic if water is repelledfrom this material or surface and that a droplet of water placed on ahydrophobic surface or material will form a large contact angle. Asurface or a material is said to be oleophilic if it has a strongaffinity for oils and not for water. The present method allows forprecise control over hydrophobicity, hydrophilicity, and oleophilicity.

The present invention also provides an oil-removing, oil-separating, oroil-recovering device, which contains the presently invented 3Dgraphene-carbon hybrid foam as an oil-absorbing or oil-separatingelement. Also provided is a solvent-removing or solvent-separatingdevice containing the 3D graphene-carbon hybrid foam as asolvent-absorbing element.

A major advantage of using the instant graphene-carbon hybrid foam as anoil-absorbing element is its structural integrity. Due to the notionthat graphene sheets are chemically bonded by the carbon material, theresulting foam would not get disintegrated upon repeated oil absorptionoperations. In contrast, we have discovered that graphene-basedoil-absorbing elements prepared by hydrothermal reduction,vacuum-assisted filtration, or freeze-drying get disintegrated afterabsorbing oil for 2 or 3 times. There is just nothing (other than weakvan der Waals forces existing prior to first contact with oil) to holdthese otherwise separated graphene sheets together. Once these graphenesheets are wetted by oil, they no longer are able to return to theoriginal shape of the oil-absorbing element.

Another major advantage of the instant technology is the flexibility indesigning and making oil-absorbing elements that are capable ofabsorbing oil up to an amount as high as 400 times of its own weight yetstill maintaining its structural shape (without significant expansion).This amount depends upon the specific pore volume of the foam, which canbe controlled mainly by the ratio between the amount of original carrierpolymer particles and the amount of graphene sheets prior to the heattreatment.

The invention also provides a method to separate/recover oil from anoil-water mixture (e.g. oil-spilled water or waste water from oil sand).The method comprises the steps of (a) providing an oil-absorbing elementcomprising an integral graphene-carbon hybrid foam; (b) contacting anoil-water mixture with the element, which absorbs the oil from themixture; and (c) retreating the oil-absorbing element from the mixtureand extracting the oil from the element. Preferably, the methodcomprises a further step of (d) reusing the element.

Additionally, the invention provides a method to separate an organicsolvent from a solvent-water mixture or from a multiple-solvent mixture.The method comprises the steps of (a) providing an organicsolvent-absorbing element comprising an integral graphene-carbon hybridfoam; (b) bringing the element in contact with an organic solvent-watermixture or a multiple-solvent mixture containing a first solvent and atleast a second solvent; (c) allowing this element to absorb the organicsolvent from the mixture or absorb the first solvent from the at leastsecond solvent; and (d) retreating the element from the mixture andextracting the organic solvent or first solvent from the element.Preferably, the method contains an additional step (e) of reusing thesolvent-absorbing element.

The following examples are used to illustrate some specific detailsabout the best modes of practicing the instant invention and should notbe construed as limiting the scope of the invention.

Example 1: Production of Graphene-Carbon Foam from Flake Graphite ViaPolypropylene Powder-Based Solid Polymer Carrier

In an experiment, 1 kg of polypropylene (PP) pellets, 50 grams of flakegraphite, 50 mesh (average particle size 0.18 mm; Asbury Carbons, AsburyN.J.) and 250 grams of magnetic steel balls were placed in a high-energyball mill container. The ball mill was operated at 300 rpm for 2 hours.The container lid was removed and stainless steel balls were removed viaa magnet. The polymer carrier material was found to be coated with adark graphene layer. Carrier material was placed over a 50 mesh sieveand a small amount of unprocessed flake graphite was removed.

A sample of the coated carrier material was then immersed intetrachloroethylene at 80° C. for 24 hours to dissolve PP and allowgraphene sheets to disperse in the organic solvent. After solventremoval, isolated graphene sheet powder was recovered (mostly few-layergraphene). The remaining coated carrier material was then compacted in amold cavity to form a green compact, which was then heat-treated in asealed crucible at 350° C. and then at 600° C. for 2 hours to produce agraphene-carbon foam.

In a separate experiment, the same batch of PP pellets and flakegraphite particles (without the impacting steel balls) were placed inthe same high-energy ball mill container and the ball mill was operatedunder the same conditions for the same period of time. The results werecompared with those obtained from impacting ball-assisted operation. Theisolated graphene sheets isolated from PP particles, upon PPdissolution, are mostly single-layer graphene. The graphene-carbon foamproduced from this process has a higher level of porosity (lowerphysical density).

Although polypropylene (PP) is herein used as an example, the carriermaterial for graphene-carbon hybrid foam production is not limited toPP. It could be any polymer (thermoplastic, thermoset, rubber, wax,mastic, gum, organic resin, etc.) provided the polymer can be made intoa particulate form. It may be noted that un-cured or partially curedthermosetting resins (such as epoxide and imide-based oligomers orrubber) can be made into a particle form at room temperature or lower(e.g. cryogenic temperature). Hence, even partially cured thermosettingresin particles can be used as a polymer carrier.

Example 2: Graphene-Carbon Hybrid Foam Using Expanded Graphite (>100 nmin Thickness) as the Graphene Source and ABS as the Polymer SolidCarrier Particles

In an experiment, 100 grams of ABS pellets, as solid carrier materialparticles, were placed in a 16 oz plastic container along with 5 gramsof expanded graphite. This container was placed in an acoustic mixingunit (Resodyn Acoustic mixer) and processed for 30 minutes. Afterprocessing, carrier material was found to be coated with a thin layer ofcarbon. A small sample of carrier material was placed in acetone andsubjected to ultrasound energy to speed dissolution of the ABS. Thesolution was filtered using an appropriate filter and washed four timeswith additional acetone. Subsequent to washing, filtrate was dried in avacuum oven set at 60° C. for 2 hours. This sample was examined byoptical microscopy and found to be graphene. The remaining pellets wereextruded to create graphene-polymer sheets (1 mm thick), which were thencarbonized to prepare graphene-carbon foam samples under differenttemperature and compression conditions.

Example 3: Production of Graphene-Carbon Hybrid Foam from Meso-CarbonMicro Beads (MCMBs as the Graphene Source Material)) andPolyacrylonitrile (PAN) Fibers (as Solid Carrier Particles)

In one example, 100 grams of PAN fiber segments (2 mm long as thecarrier particles), 5 grams of MCMBs (China Steel Chemical Co., Taiwan),and 50 grams of zirconia beads were placed in a vibratory ball mill andprocessed for 2 hours. After the process was completed, the vibratorymill was then opened and the carrier material was found to be coatedwith a dark coating of graphene sheets. The zirconia particles, havingdistinctly different sizes and colors were manually removed. Thegraphene-coated PAN fibers were then compacted and melted together toform several composite films. The films were subjected to a heattreatment at 250° C. for 1 hour (in room air), 350° C. for 2 hours, and1,000° C. for 2 hours (under an argon gas atmosphere) to obtaingraphene-carbon foam layers. Half of the carbonized foam layers werethen heated to 2,850° C. and maintained at this temperature for 0.5hours.

Example 4: Particles of Cured Phenolic Resin as the Polymer Carrier in aFreezer Mill

In one experiment, 10 grams of phenolic resin particles were placed in aSPEX mill sample holder (SPEX Sample Prep, Metuchen, N.J.) along with0.25 grams of HOPG powder derived from graphitized polyimide and amagnetic stainless steel impactor. The same experiment was performed,but the sample holder did not contain any impactor balls. Theseprocesses were carried out in a 1%-humidity “dry room” to reduce thecondensation of water onto the completed product. The SPEX mill wasoperated for 10-120 minutes. After operation, the contents of the sampleholder were sorted to recover graphene-coated resin particles byremoving residual HOPG powder and impactor balls (when used).

The resulting graphene-coated resin particles in both cases (with orwithout impactor balls) were examined using both digital opticalmicroscopy and scanning electron microscopy (SEM). It was observed thatthe thickness of the graphene sheets wrapped around resin particlesincreases with the milling operation time and, given the same durationof operation, the impactor-assisted operation leads to thicker graphenecoating.

A mass of graphene-coated resin particles was compressed to form a greencompact, which was then infiltrated with a small amount of petroleumpitch. Separately, another green compact of graphene-coated resinparticles was prepared under comparable conditions, but no pitchinfiltration was attempted. The two compacts were then subjected toidentical pyrolysis treatments.

Example 5: Natural Graphite Particles as the Graphene Source,Polyethylene (PE) or Nylon 6/6 Beads as the Solid Carrier Particles, andCeramic or Glass Beads as Added Impacting Balls

In an experiment, 0.5 kg of PE or nylon beads (as a solid carriermaterial), 50 grams of natural graphite (source of graphene sheets) and250 grams of zirconia powder (impacting balls) were placed in containersof a planetary ball mill. The ball mill was operated at 300 rpm for 4hours. The container lid was removed and zirconia beads (different sizesand weights than graphene-coated PE beads) were removed through avibratory screen. The polymer carrier material particles were found tobe coated with a dark graphene layer. Carrier material was placed over a50 mesh sieve and a small amount of unprocessed flake graphite wasremoved. In a separate experiment, glass beads were used as theimpacting balls; other ball-milling operation conditions remained thesame.

A mass of graphene-coated PE pellets and a mass of graphene-coated nylonbeads were separately compacted in a mold cavity and briefly heatedabove the melting point of PE or nylon and then rapidly cooled to formtwo green compacts. For comparison purposes, two corresponding compactswere prepared from a mass of un-coated PE pellets and a mass ofun-coated nylon beads. These 4 compacts were then subjected topyrolyzation (by heating the compacts in a chamber from 100° C. to 650°C.). The results were very surprising. The compacts of graphene-coatedpolymer particles were found to be converted to graphene-carbon hybridfoam structures having dimensions comparable to the dimensions of theoriginal compacts (3 cm×3 cm×0.5 cm). SEM examination of thesestructures indicates that carbon phases are present near the edges ofgraphene sheets and these carbon phases act to bond the graphene sheetstogether. The carbon-bonded graphene sheets form a skeleton ofgraphene-carbon hybrid pore walls having pores being present in whatused to be the space occupied by the original polymer particles, asschematically illustrated in FIG. 2(A).

In contrast, the two compacts from un-coated pellets or beads shrank tobecome essentially two solid masses of carbon having a volumeapproximately 15%-20% of the original compact volumes. These highlyshrunk solid masses are practically pore-free carbon materials; they arenot a foam material.

Examples 6: Micron-Sized Rubber Particles as the Solid Polymer CarrierParticles

The experiment began with preparation of micron-sized rubber particles.A mixture of methylhydro dimethyl-siloxane polymer (20 g) andpolydimethylsiloxane, vinyldimethyl terminated polymer (30 g) wasobtained by using a homogenizer under ambient conditions for 1 minute.Tween 80 (4.6 g) was added and the mixture was homogenized for 20seconds. Platinum-divinyltetramethyldisiloxane complex (0.5 g in 15 gmethanol) was added and mixed for 10 seconds. This mixture was added to350 g of distilled water and a stable latex was obtained byhomogenization for 15 minutes. The latex was heated to 60° C. for 15hours. The latex was then de-emulsified with anhydrous sodium sulfate(20 g) and the silicone rubber particles were obtained by filtrationunder a vacuum, washing with distilled water, and drying under vacuum at25° C. The particle size distribution of the resulting rubber particleswas 3-11 μm.

In one example, 10 grams of rubber particles, 2 grams of naturalgraphite, and 5 grams of zirconia beads were placed in a vibratory ballmill and processed for 2 hours. After the process was completed, thevibratory mill was then opened and the rubber particles were found to becoated with a dark coating of graphene sheets. The zirconia particleswere manually removed. The graphene-coated rubber particles were thenmixed with 5% by wt. of petroleum pitch (as a binder) and mechanicallycompacted together to form several composite sheets. The compositesheets were then subjected to a heat treatment at 350° C. for 1 hour,650° C. for 2 hours, and 1,000° C. for 1 hour in a tube furnace toobtain graphene-carbon foam layers.

Examples 7: Preparation of Graphene Fluoride Foams

In a typical procedure, a sheet of graphene-carbon hybrid wasfluorinated by vapors of chlorine trifluoride in a sealed autoclavereactor to yield fluorinated graphene-carbon hybrid film. Differentdurations of fluorination time were allowed for achieving differentdegrees of fluorination. Sheets of fluorinated graphene-carbon foam werethen separately immersed in containers each containing achloroform-water mixture. We observed that these foam sheets selectivelyabsorb chloroform from water and the amount of chloroform absorbedincreases with the degree of fluorination until the fluorine contentreaches 7.3% by wt. (FIG. 9)

Example 8: Preparation of Graphene Oxide Foam and Nitrogenated GrapheneFoams

Several pieces of graphene-carbon foam prepared in Example 3 wereimmersed in a 30% H₂O₂-water solution for a period of 2-48 hours toobtain graphene oxide (GO) foams, having an oxygen content of 2-25% byweight.

Some GO foam samples were mixed with different proportions of urea andthe mixtures were heated in a microwave reactor (900 W) for 0.5 to 5minutes. The products were washed several times with deionized water andvacuum dried. The products obtained were nitrogenated graphene foam. Thenitrogen contents were from 3% to 17.5 wt. %, as measured by elementalanalysis.

It may be noted that different functionalization treatments of thegraphene-carbon hybrid foam were for different purposes. For instance,oxidized graphene-carbon hybrid foam structures are particularlyeffective as an absorber of oil from an oil-water mixture (i.e. oilspilled on water and then mixed together). In this case, the integral 3Dgraphene (0-15% by wt. oxygen)-carbon foam structures are bothhydrophobic and oleophilic (FIG. 7). A surface or a material is said tobe hydrophobic if water is repelled from this material or surface andthat a droplet of water placed on a hydrophobic surface or material willform a large contact angle. A surface or a material is said to beoleophilic if it has a strong affinity for oils and not for water.

Different contents of O, F, and/or N also enable the presently inventedgraphene-carbon hybrid foams to absorb different organic solvents fromwater, or to separate one organic solvent from a mixture of multiplesolvents.

Comparative Example 1: Graphene Via Hummer's Process and Carbonizationof Graphene-Polymer Composite

Graphite oxide as prepared by oxidation of graphite flakes with sulfuricacid, nitrate, and permanganate according to the method of Hummers [U.S.Pat. No. 2,798,878, Jul. 9, 1957]. Upon completion of the reaction, themixture was poured into deionized water and filtered. The graphite oxidewas repeatedly washed in a 5% solution of HCl to remove most of thesulphate ions. The sample was then washed repeatedly with deionizedwater until the pH of the filtrate was neutral. The slurry wasspray-dried and stored in a vacuum oven at 60° C. for 24 hours. Theinterlayer spacing of the resulting laminar graphite oxide wasdetermined by the Debey-Scherrer X-ray technique to be approximately0.73 nm (7.3 A). A sample of this material was subsequently transferredto a furnace pre-set at 650° C. for 4 minutes for exfoliation and heatedin an inert atmosphere furnace at 1200° C. for 4 hours to create a lowdensity powder comprised of few-layer reduced graphene oxide (RGO).Surface area was measured via nitrogen adsorption BET. This powder wassubsequently dry mixed at a 1%-25% loading level with ABS, PE, PP, andnylon pellets, respectively, and compounded using a 25 mm twin screwextruder to form composite sheets. These composite sheets were thenpyrolyzed.

Comparative Example 2: Preparation of Single-Layer Graphene Oxide (GO)Sheets from Meso-Carbon Micro-Beads (MCMBs) and then Production ofGraphene Foam Layers from GO Sheets

Meso-carbon microbeads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. MCMB (10 grams) wereintercalated with an acid solution (sulfuric acid, nitric acid, andpotassium permanganate at a ratio of 4:1:0.05) for 48-96 hours. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The intercalated MCMBs were repeatedly washed in a 5%solution of HCl to remove most of the sulphate ions. The sample was thenwashed repeatedly with deionized water until the pH of the filtrate wasno less than 4.5. The slurry was then subjected ultrasonication for10-100 minutes to produce GO suspensions. TEM and atomic forcemicroscopic studies indicate that most of the GO sheets weresingle-layer graphene when the oxidation treatment exceeded 72 hours,and 2- or 3-layer graphene when the oxidation time was from 48 to 72hours.

The GO sheets contain oxygen proportion of approximately 35%-47% byweight for oxidation treatment times of 48-96 hours. GO sheets weresuspended in water. Baking soda (5-20% by weight), as a chemical blowingagent, was added to the suspension just prior to casting. The suspensionwas then cast onto a glass surface. Several samples were cast, somecontaining a blowing agent and some not. The resulting GO films, afterremoval of liquid, have a thickness that can be varied fromapproximately 10 to 500 μm. Several sheets of the GO film, with orwithout a blowing agent, were then subjected to heat treatments thatinvolve a heat temperature of 80-500° C. for 1-5 hours, which generateda graphene foam structure.

Comparative Example 3: Preparation of Pristine Graphene Foam (0% Oxygen)

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free, non-halogenated and halogen-free, etc.) can lead to agraphene foam having a higher thermal conductivity. Pristine graphenesheets were produced by using the direct ultrasonication process (alsoknown as the liquid-phase exfoliation in the art).

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson S450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.There are no other non-carbon elements.

Various amounts (1%-30% by weight relative to graphene material) ofchemical bowing agents (N, N-Dinitroso pentamethylene tetramine or 4,4′-Oxybis (benzenesulfonyl hydrazide) were added to a suspensioncontaining pristine graphene sheets and a surfactant. The suspension wasthen cast onto a glass surface. Several samples were cast, including onethat was made using CO₂ as a physical blowing agent introduced into thesuspension just prior to casting. The resulting graphene films, afterremoval of liquid, have a thickness that can be varied fromapproximately 10 to 100 μm. The graphene films were then subjected toheat treatments at a temperature of 80-1,500° C. for 1-5 hours, whichgenerated a graphene foam.

Comparative Example 4: CVD Graphene Foams on Ni Foam Templates

The procedure was adapted from that disclosed in open literature: Chen,Z. et al. “Three-dimensional flexible and conductive interconnectedgraphene networks grown by chemical vapor deposition,” Nat. Mater. 10,424-428 (2011). Nickel foam, a porous structure with an interconnected3D scaffold of nickel was chosen as a template for the growth ofgraphene foam. Briefly, carbon was introduced into a nickel foam bydecomposing CH₄ at 1,000° C. under ambient pressure, and graphene filmswere then deposited on the surface of the nickel foam. Due to thedifference in the thermal expansion coefficients between nickel andgraphene, ripples and wrinkles were formed on the graphene films. Inorder to recover (separate) graphene foam, Ni frame must be etched away.Before etching away the nickel skeleton by a hot HCl (or FeCl₃)solution, a thin layer of poly(methyl methacrylate) (PMMA) was depositedon the surface of the graphene films as a support to prevent thegraphene network from collapsing during nickel etching. After the PMMAlayer was carefully removed by hot acetone, a fragile graphene foamsample was obtained. The use of the PMMA support layer is critical topreparing a free-standing film of graphene foam; only a severelydistorted and deformed graphene foam sample was obtained without thePMMA support layer. This is a tedious process that is notenvironmentally benign and is not scalable.

Comparative Example 5: Conventional Graphitic Foam from Pitch-BasedCarbon Foams

Pitch powder, granules, or pellets are placed in a aluminum mold withthe desired final shape of the foam. Mitsubishi ARA-24 meso-phase pitchwas utilized. The sample is evacuated to less than 1 torr and thenheated to a temperature approximately 300° C. At this point, the vacuumwas released to a nitrogen blanket and then a pressure of up to 1,000psi was applied. The temperature of the system was then raised to 800°C. This was performed at a rate of 2 degree C./min. The temperature washeld for at least 15 minutes to achieve a soak and then the furnacepower was turned off and cooled to room temperature at a rate ofapproximately 1.5 degree C./min with release of pressure at a rate ofapproximately 2 psi/min. Final foam temperatures were 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C.(carbonized) under a nitrogen blanket and then heat treated in separateruns in a graphite crucible to 2500° C. and 2800° C. (graphitized) inArgon.

Comparative Example 6: Graphene Foams from Hydrothermally ReducedGraphene Oxide

For comparison, a self-assembled graphene hydrogel (SGH) sample wasprepared by a one-step hydrothermal method. In a typical procedure, theSGH can be easily prepared by heating 2 mg/mL of homogeneous grapheneoxide (GO) aqueous dispersion sealed in a Teflon-lined autoclave at 180°C. for 12 h. The SGH containing about 2.6% (by weight) graphene sheetsand 97.4% water has an electrical conductivity of approximately 5×10⁻³S/cm. Upon drying and heat treating at 1,500° C., the resulting graphenefoam exhibits an electrical conductivity of approximately 1.5×10⁻¹ S/cm,which is 2 times lower than those of the presently invented graphenefoams produced by heat treating at the same temperature.

Example 9: Thermal and Mechanical Testing of Various Graphene Foams andConventional Graphite Foam

Samples from various conventional carbon or graphene foam materials weremachined into specimens for measuring the thermal conductivity. The bulkthermal conductivity of meso-phase pitch-derived foam ranged from 67W/mK to 151 W/mK. The density of the samples was from 0.31-0.61 g/cm³.When weight is taken into account, the specific thermal conductivity ofthe pitch derived foam is approximately 67/0.31=216 and 151/0.61=247.5W/mK per specific gravity (or per physical density).

The compression strength of the samples having an average density of0.51 g/cm³ was measured to be 3.6 MPa and the compression modulus wasmeasured to be 74 MPa. By contrast, the compression strength andcompressive modulus of the presently invented graphene-carbon foamsamples having a comparable physical density are 6.2 MPa and 113 MPa,respectively.

Shown in FIG. 4(A) are the thermal conductivity values vs. specificgravity of the 3D graphene-carbon foam, meso-phase pitch-derivedgraphite foam, and Ni foam template-assisted CVD graphene foam. Thesedata clearly demonstrate the following unexpected results:

-   -   1) The 3D integral graphene-carbon foams produced by the        presently invented process exhibit significantly higher thermal        conductivity as compared to both meso-phase pitch-derived        graphite foam and Ni foam template-assisted CVD graphene, given        the same physical density.    -   2) This is quite surprising in view of the notion that CVD        graphene is essentially pristine graphene that has never been        exposed to oxidation and should have exhibited a high thermal        conductivity compared to our graphene-carbon hybrid foam. The        carbon phase of the hybrid foam is in general of low degree of        crystallinity (some being amorphous carbon) and, thus, has much        lower thermal or electrical conductivity as compared with        graphene alone. However, when the carbon phase is coupled with        graphene sheets to form an integral structure produced by the        presently invented method, the resulting hybrid form exhibits a        thermal conductivity as compared to an all-pristine graphene        foam. These exceptionally high thermal conductivity values        observed with the graphene-carbon hybrid foams herein produced        are much to our surprise. This is likely due to the observation        that the otherwise isolated graphene sheets are now bonded by a        carbon phase, providing a bridge for the uninterrupted transport        of electrons and phonons.    -   3) The specific conductivity values of the presently invented        hybrid foam materials exhibit values from 250 to 500 W/mK per        unit of specific gravity; but those of other types of foam        materials are typically lower than 250 W/mK per unit of specific        gravity.    -   4) Summarized in FIG. 5 are thermal conductivity data for a        series of 3D graphene-carbon foams and a series of pristine        graphene derived foams, both plotted over the final (maximum)        heat treatment temperatures. In both types of materials, the        thermal conductivity increases monotonically with the final HTT.        However, the presently invented process enables the        cost-effective and environmentally benign production of        graphene-carbon foams that outperform pristine graphene foams.        This is another unexpected result.    -   5) FIG. 4(B) shows the thermal conductivity values of the        presently invented hybrid foam and hydrothermally reduced GO        graphene foam. Electrical conductivity values of 3D        graphene-carbon foam and the hydrothermally reduced GO graphene        foam are shown in FIG. 6. These data further support the notion        that, given the same amount of solid material, the presently        invented graphene-carbon foam is intrinsically most conducting,        reflecting the significance of continuity in electron and phonon        transport paths. The carbon phase bridges the gaps or        interruptions between graphene sheets.

Example 10: Characterization of Various Graphene Foams and ConventionalGraphite Foam

The internal structures (crystal structure and orientation) of severalseries of graphene-carbon foam materials were investigated using X-raydiffraction. The X-ray diffraction curve of natural graphite typicallyexhibits a peak at approximately 2θ=26°, corresponds to aninter-graphene spacing (d₀₀₂) of approximately 0.3345 nm. The graphenewalls of the hybrid foam materials exhibit a d₀₀₂ spacing typically from0.3345 nm to 0.40 nm, but more typically up to 0.34 nm.

With a heat treatment temperature of 2,750° C. for the foam structureunder compression for one hour, the d₀₀₂ spacing is decreased toapproximately to 0.3354 nm, identical to that of a graphite singlecrystal. In addition, a second diffraction peak with a high intensityappears at 2θ=55° corresponding to X-ray diffraction from (004) plane.The (004) peak intensity relative to the (002) intensity on the samediffraction curve, or the I(004)/I(002) ratio, is a good indication ofthe degree of crystal perfection and preferred orientation of grapheneplanes. The (004) peak is either non-existing or relatively weak, withthe I(004)/I(002) ratio <0.1, for all graphitic materials heat treatedat a temperature lower than 2,800° C. The I(004)/I(002) ratio for thegraphitic materials heat treated at 3,000-3,250° C. (e.g., highlyoriented pyrolytic graphite, HOPG) is in the range of 0.2-0.5. Incontrast, a graphene foam prepared with a final HIT of 2,750° C. for onehour exhibits a I(004)/I(002) ratio of 0.78 and a Mosaic spread value of0.21, indicating the pore walls being a practically perfect graphenesingle crystal with a good degree of preferred orientation (if preparedunder a compression force).

The “mosaic spread” value is obtained from the full width at halfmaximum of the (002) reflection in an X-ray diffraction intensity curve.This index for the degree of ordering characterizes the graphite orgraphene crystal size (or grain size), amounts of grain boundaries andother defects, and the degree of preferred grain orientation. A nearlyperfect single crystal of graphite is characterized by having a mosaicspread value of 0.2-0.4. Some of our graphene foams have a mosaic spreadvalue in this range of 0.3-0.6 when produced using a final heattreatment temperature no less than 2,500° C.

The following are a summary of some of the more significant results:

-   -   1) In general, the addition of impacting balls helps to        accelerate the process of peeling off graphene sheets from        graphite particles. However, this option necessitates the        separation of these impacting balls after graphene-coated        polymer particles are made.    -   2) When no impacting balls (e.g. ceramic, glass, metal balls,        etc.) are used, harder polymer particles (e.g. PE, PP, nylon,        ABS, polystyrene, high impact polystyrene, etc. and their        filler-reinforced versions) are more capable of peeling off        graphene sheets from graphite particles, as compared to softer        polymer particles (e.g. rubber, PVC, polyvinyl alcohol, latex        particles).    -   3) Without externally added impacting balls, softer polymer        particles tend to result in graphene-coated or embedded        particles having 0.001% to 5% by weight of graphene (mostly        single-layer graphene sheets) and harder polymer balls tend to        lead to graphene-coated particles having 0.01% to 30% by weight        of graphene (mostly single-layer and few layer graphene sheets),        given the same 1 hour of operating time.    -   4) With externally added impacting balls, all polymer balls are        capable of supporting from 0.001% to approximately 80% by weight        of graphene sheets (mostly few-layer graphene, <10 layers, if        over 30% by weight of graphene sheets).    -   5) The presently invented graphene-carbon hybrid foam materials        typically exhibit significantly higher structural integrity        (e.g. compression strength, elasticity, and resiliency) and        higher thermal and electrical conductivities as compared to        their counterparts produced by the conventional, prior art        methods.    -   6) It is of significance to point out that all the prior art        processes for producing graphite foams or graphene foams appear        to provide only macro-porous foams having a physical density in        the range of approximately 0.2-0.6 g/cm³, with pore sizes being        typically too large (e.g. from 20 to 300 μm) for most of the        intended applications. In contrast, the instant invention        provides processes that generate graphene foams having a density        that can be as low as 0.001 g/cm³ and as high as 1.7 g/cm³. The        pore sizes can be varied from microscopic (<2 nm), through        meso-scaled (2-50 nm), and up to macro-scaled (e.g. from 1 to        500 μm). This level of flexibility and versatility in designing        various types of graphene-carbon foams is unprecedented and        un-matched by any prior art process.    -   7) The presently invented method also allows for convenient and        flexible control over the chemical composition (e.g. F, O, and N        contents, etc.), responsive to various application needs (e.g.        oil recovery from oil-contaminated water, separation of an        organic solvent from water or other solvents, heat dissipation,        etc.).        In conclusion, we have successfully developed an absolutely new,        novel, unexpected, and patently distinct class of highly        conducting graphene-carbon hybrid foam materials, devices, and        related processes of production. The chemical composition (% of        oxygen, fluorine, and other non-carbon elements), structure        (crystal perfection, grain size, defect population, etc.),        crystal orientation, morphology, process of production, and        properties of this new class of foam materials are fundamentally        different and patently distinct from meso-phase pitch-derived        graphite foam, CVD graphene-derived foam, and graphene foams        from hydrothermal reduction of GO.

We claim:
 1. An integral 3D graphene-carbon hybrid foam composed ofmultiple pores and pore walls, wherein said pore walls containsingle-layer or few-layer graphene sheets chemically bonded by a carbonmaterial having a carbon material-to-graphene weight ratio from 1/200 to1/2, wherein said few-layer graphene sheets have 2-10 layers of stackedgraphene planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to0.40 nm as measured by X-ray diffraction and said single-layer orfew-layer graphene sheets contain a pristine graphene material havingessentially zero % of non-carbon elements, or a non-pristine graphenematerial having 0.001% to 25% by weight of non-carbon elements whereinsaid non-pristine graphene is selected from graphene oxide, reducedgraphene oxide, graphene fluoride, graphene chloride, graphene bromide,graphene iodide, hydrogenated graphene, nitrogenated graphene, dopedgraphene, chemically functionalized graphene, or a combination thereof.2. The integral 3D graphene-carbon hybrid foam of claim 1, wherein said3D graphene foam has a density from 0.005 to 1.7 g/cm³, a specificsurface area from 50 to 3,200 m²/g, a thermal conductivity of at least200 W/mK per unit of specific gravity, and/or an electrical conductivityno less than 2,000 S/cm per unit of specific gravity.
 3. The integral 3Dgraphene-carbon hybrid foam of claim 1, wherein said pore walls containa pristine graphene and said 3D graphene-carbon hybrid foam has adensity from 0.01 to 1.7 g/cm³ or an average pore size from 2 nm to 50nm.
 4. The integral 3D graphene-carbon hybrid foam of claim 1, whereinsaid pore walls contain a non-pristine graphene material and whereinsaid foam contains a content of non-carbon elements in the range of0.01% to 20% by weight and said non-carbon elements include an elementselected from oxygen, fluorine, chlorine, bromine, iodine, nitrogen,hydrogen, or boron.
 5. The integral 3D graphene-carbon hybrid foam ofclaim 1, wherein said pore walls contain graphene fluoride and saidsolid graphene foam contains a fluorine content from 0.01% to 15% byweight.
 6. The 3D graphene-carbon hybrid foam of claim 1, wherein saidpore walls contain graphene oxide and said solid graphene foam containsan oxygen content from 0.01% to 20% by weight.
 7. The 3D graphene-carbonhybrid foam of claim 1, wherein said foam has a specific surface areafrom 200 to 3,000 m²/g or a density from 0.1 to 1.2 g/cm³.
 8. The 3Dgraphene-carbon hybrid foam of claim 1, which is in a continuous-lengthroll sheet form having a thickness from 100 nm to 10 cm and a length ofat least 2 meters and is produced by a roll-to-roll process.
 9. The 3Dgraphene-carbon hybrid foam of claim 1, wherein said foam has an oxygencontent or non-carbon content less than 1% by weight, and said porewalls have an inter-graphene spacing less than 0.35 nm, a thermalconductivity of at least 250 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 2,500 S/cm per unit of specificgravity.
 10. The 3D graphene-carbon hybrid foam of claim 1, wherein saidfoam has an oxygen content or non-carbon content less than 0.01% byweight and said pore walls contain stacked graphene planes having aninter-graphene spacing less than 0.34 nm, a thermal conductivity of atleast 300 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 3,000 S/cm per unit of specific gravity. 11.The 3D graphene foam of claim 1, wherein said foam has an oxygen contentor non-carbon content no greater than 0.01% by weight and said porewalls contain stacked graphene planes having an inter-graphene spacingless than 0.336 nm, a thermal conductivity of at least 350 W/mK per unitof specific gravity, and/or an electrical conductivity no less than3,500 S/cm per unit of specific gravity.
 12. The 3D graphene-carbonhybrid foam of claim 1, wherein said foam has pore walls containingstacked graphene planes having an inter-graphene spacing less than 0.336nm, a thermal conductivity greater than 400 W/mK per unit of specificgravity, and/or an electrical conductivity greater than 4,000 S/cm perunit of specific gravity.
 13. The 3D graphene-carbon hybrid foam ofclaim 1, wherein the pore walls contain stacked graphene planes havingan inter-graphene spacing less than 0.337 nm and a mosaic spread valueless than 1.0.
 14. The 3D graphene-carbon hybrid foam of claim 1,wherein said pore walls contain a 3D network of interconnected grapheneplanes.
 15. The 3D graphene-carbon hybrid foam of claim 1, wherein saidfoam contains meso-scaled pores having a pore size from 2 nm to 50 nm.16. An oil-removing or oil-separating device containing the 3Dgraphene-carbon hybrid foam of claim 1 as an oil-absorbing element. 17.A solvent-removing or solvent-separating device containing the 3Dgraphene-carbon hybrid foam of claim 1 as a solvent-absorbing orsolvent-separating element.
 18. A method to separate oil from water,said method comprising the steps of: a. Providing an oil-absorbingelement comprising the integral 3D graphene-carbon hybrid foam of claim1; b. Contacting an oil-water mixture with said element, which absorbsthe oil from the mixture; c. Retreating the element from the mixture andextracting the oil from the element; and d. Reusing the element.
 19. Amethod to separate an organic solvent from a solvent-water mixture orfrom a multiple-solvent mixture, said method comprising the steps of: a.Providing an organic solvent-absorbing or solvent-separating elementcomprising the integral 3D graphene-carbon hybrid foam of claim 1; b.Bringing said element in contact with an organic solvent-water mixtureor a multiple-solvent mixture containing a first solvent and at least asecond solvent; c. Allowing said element to absorb the organic solventfrom the mixture or separate said first solvent from said at leastsecond solvent; d. Retreating the element from the mixture andextracting the organic solvent or first solvent from the element; and e.Reusing the element.
 20. A thermal management device containing the 3Dintegral graphene-carbon hybrid foam of claim 1 as a heat spreading orheat dissipating element.
 21. The thermal management device of claim 20,which contains a device selected from a heat exchanger, heat sink, heatpipe, high-conductivity insert, conductive plate between a heat sink anda heat source, heat-spreading component, heat-dissipating component,thermal interface medium, or thermoelectric or Peltier cooling device.